Absorption spectra of Co 2+ in AgInS 2 orthorhombic single crystals
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Seok-Kyun Oh and Wha-Tek Kim Korea Basic Science Institute and Department of Physics, Chonnam National University, Kwangju 500-757, Republic of Korea (Received 01 December 2000; accepted 14 January 2001)
The optical absorption spectra of AgInS2:Co2+ single crystals grown by chemical transport reaction using iodine as a transporting medium have been studied at 6 K. The peaks can be explained by the transitions of Co2+ in the Td symmetry with the spin-orbit coupling, which means that the deviations of the atomic sites from those of the ideal wurtzite structure can be considered negligibly small. The consideration of both the crystal field parameters and the electronegativity difference between atoms may indicate that Co atoms substitute In atoms.
I. INTRODUCTION
The family of ternary semiconductors is of great interest because of their practical applications, particularly in light-emitting diodes, electro and nonlinear optics, and the study of their various physical properties. The AgInS2 compound is unique among the ternary compounds in that it exists at atmosphere pressure and room temperature in two ordered phases: along with the tetragonal chalcopyrite structure there is an orthorhombic wurtzite-like one.1 The orthorhombic modification of AgInS2 is pseudo-wurtzite, just as the chalcopyrite structure is pseudo-zinc-blende.2 The atomic site positions in the former are very nearly the same as in wurtzite. However, an ordering of Ag and In atoms on the cation sublattice results in a doubling of the unit cell dimension and reduces the symmetry, thereby permitting an internal orthorhombic distortion with space group Pna21(C 92v).3 This space group has no point symmetry at atoms’ sites. From these facts, this structure is like Fig. 1. There are two distinct types of distortion from the ideal wurtzite structure, where a cation is surrounded by a regular tetrahedron (Td) of four anions: Each anion is displaced from its wurtzite site (anion displacement) and the lattice constant ratio deviates from the ideal value, √3:2:√8/3 (orthorhombic distortion).2 So, the symmetry of anion atoms around each cation is lowered from Td to C1.
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Present address: Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. J. Mater. Res., Vol. 16, No. 4, Apr 2001
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Energy levels of 3d transition-metal (TM) impurities in semiconducting materials are split under the influence of the crystal field. Studies on these energy-level splittings are useful tools to understand the crystal structure and the crystal-field strength of the host lattice.4,5 The energy levels of Co2+ in the orthorhombic AgInS2 may be split under the crystal field of the Td, and C1 symmetry along with the spin-orbit coupling interaction. Reduction of the crystal symmetry may further split the energy levels of cobalt impurity. The splitting can be derived from the crystal-field theory considering group theoretical calculation, which uses character tables for point-symm
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